1. Field of the Invention
The present invention relates to an optical apparatus configured to drive an optical member by using a stepping motor.
2. Description of the Related Art
Recently, an autofocus detector has been used, as an essential component, in an image pickup apparatus, such as a general video camera. Further, precise position control is demanded in driving a focusing mechanism when focus adjustment is performed. A stepping motor is often used as a driving source for driving the focusing mechanism. Because the stepping motor is rotated one-step by one-step in accordance with a driving command of one pulse, driving steps can be predicted at the time when the driving command is issued. In general, the focusing mechanism using the stepping motor, as the driving source, is constructed of a simple open-loop control system in which feedback control is not performed.
However, a position of the focusing mechanism, which is detected by the open loop control, is sometimes deviated from an actual position due to an influence of backlash, such as a reverse hysteresis and a play, which is present in a transmission mechanism for transmitting motive power of the stepping motor to the focusing mechanism. Accordingly, when highly-accurate position control is needed, the feedback control is performed by using an MR (Magnetic Resistance) sensor or the like, as a position sensor in the focusing mechanism, in some cases.
Japanese Patent Laid-Open No. 11-110045 discloses a usage example of combination of the stepping motor and the position sensor. According to Japanese Patent Laid-Open No. 11-110045, the position sensor is attached to a control target and feedback control is performed based on information from the position sensor, thus ensuring highly-accurate control. Further, determination as to loss of synchronism, for example, can be made based on a value of a driving command and a state of the position sensor.
With the above-cited Japanese Patent Laid-Open No. 11-110045, the driving command for the stepping motor is generated depending on the state of the position sensor. Therefore, the stepping motor cannot be stably driven in some cases due to influences of a detection delay time of the position sensor and a noise component superimposed on a signal of the position sensor. The detection delay time depends on not only a period from AD conversion of a signal of the MR sensor to recognition of position information by a microcomputer for control of the stepping motor, but also on a dead-band zone of a driving mechanism.
Thus, for example, when a lens is driven such that the position command instructing a target position of the lens and the position information from the position sensor match with each other, the lens is moved in excess of the target position and an operation of compensating for an overshoot is repeatedly generated. Hence, the so-called hunting occurs.
Further, because electric power is supplied as a rectangular-wave pulse to a driver circuit during the driving of the stepping motor, noise is more apt to mix in an analog signal. If noise mixes in the signal and the microcomputer for control of the stepping motor recognizes false position information, an unexpected malfunction may occur.